A reconfigurable active antenna with electronic beamforming comprises several radiating elements, active chains intended to process the signals received by the radiating elements, and a beamformer which recombines the signals received, in a coherent manner, in various directions to form various beams. Each radiating element is connected to the beamformer by way of a dedicated active chain. When the beamforming is carried out on microwave-frequency signals, the processings carried out by each active chain comprise a filtering and an amplification of the received signals. When the beamforming is carried out on analogue signals transposed to baseband, the processings carried out by each active chain furthermore comprise a frequency transposition. The processings can also comprise digitization if the beamforming is carried out on digitized signals.
Conventionally, as represented in the example of FIG. 1, a radiofrequency planar beamformer divides the signals received by each radiating element E1, E2, . . . , Ei, . . . , EN, into M sub-signals which are conveyed in M different channels, and then applies a phase shift and an attenuation of controllable value to each of the M sub-signals, before recombining the sub-signals originating from the N radiating elements so as to form M different beams S1, S2, . . . , SM, also called spots. However, the radiofrequency planar beamformer makes it necessary to produce crossovers between the channels conveying the sub-signals, the number of crossovers being equal to the product of the number M of beams times the number N of radiating elements. Consequently, the more significant the number of beams to be produced, the more the mass, the bulk and the complexity of this beamformer increases. This beamformer therefore quickly becomes unachievable when it is necessary to produce a large number of beams to cover a wide angular sector.
When the beamforming is carried out on analogue signals transposed to baseband, the crossovers are easier to produce by using ASICs. This makes it possible to limit the mass and the bulk of the beamformer, but this technology entails too significant a power consumption.
When the beamforming is carried out on digital signals, the digitization of the signals on a large number of radiating elements generally leads to significant consumed powers.
According to another technology, planar quasi-optical beamformers exist which use electromagnetic propagation of the radiofrequency waves originating from several feed sources placed at input, for example internal horns, according to a mode of propagation, in general TEM (Transverse Electric Magnetic), between two parallel metal plates. The focusing and the collimation of the beams can be carried out by a lens, for example an optical lens as described notably in documents U.S. Pat. No. 3,170,158 and U.S. Pat. No. 5,936,588 which illustrate the case of a Rotman lens, the lens being inserted in the propagation path of the radiofrequency waves, between the two parallel metal plates. Various types of lenses can be used, these lenses serving essentially as phase correctors and making it possible in most cases to convert a, or several, cylindrical wave transmitted by the sources into a, or several, plane wave propagating in the waveguide with parallel metal plates. The lens can comprise two opposite edges with parabolic profiles, respectively input and output. Alternatively, the lens can be a dielectric lens, or an index-gradient lens, or any other type of lens. As this technology uses parallel-plate waveguides, as alternative to the use of several discrete radiating elements aligned side by side, it is possible to use a continuous radiating linear aperture at the output of each parallel-plate waveguide. These radiating linear apertures, which are not spatially quantized, have much higher performance relative to linear arrays of several radiating elements, for squinted beams, because of the absence of quantization, and in terms of bandwidth because of the absence of resonant propagation modes.
A quasi-optical beamformer is much simpler to produce than traditional beamformers with individual waveguides since it comprises neither coupler, nor crossover device and makes it possible to produce several beams which cover a wide angular sector, without any aberration. Furthermore, their bandwidth is very significant and they can operate both in a transmission band Rx and in a reception band Tx. However, the known planar beamformers are capable of forming beams only according to a single dimension in space, in a direction parallel to the plane of the metal plates. To form beams along two dimensions in space, in two directions, respectively parallel and orthogonal to the plane of the metal plates, it is necessary to orthogonally combine together two beamforming assemblies, each beamforming assembly consisting of a stack of several layers of unidirectional beamformers. To orthogonally combine two beamforming assemblies, it is furthermore necessary to design connection interfaces, in particular input/output connectors, on each beamforming assembly and then to link the various corresponding inputs and outputs of the two beamforming assemblies pairwise by dedicated interconnection cables, as is represented for example in document U.S. Pat. No. 5,936,588 for lens-based beamformers. This architecture is satisfactory for forming a small number of beams, but becomes very complex and overly bulky when the number of beams increases.
No planar beamforming device exists which makes it possible to form beams along two dimensions in space. Moreover, neither do any simple solutions exist for interconnecting two unidirectional beamformers making it possible to dispense with the connection interfaces and interconnection cables.